In the mobile communication system using the CDMA method, signals which are received in reception side devices suffer various kinds of signal interference to cause characteristic deterioration, as signals of a plurality of users are transmitted on the same frequency band. Conventionally, there has been a device for reduction of the above interference, according to Japanese patent application No. Hei-10-126383. A conventional device for canceling interference will be described below, referring to FIG. 1.
FIG. 1 is a block diagram of a conventional device for canceling interference. In the first place, signals of all users in a unit interval are sent as reception signals to a delay device 12 and matched filters 13-1˜13-N through a switch 11. Here, a case where the above unit interval is assumed to be one slot will be described. In the delay device 12, the reception signals are sent to a subtraction device 21 described later, after they are delayed by a predetermined time.
Matched filters 13-1˜13-N perform correlation (despreading) operation on the reception signals, using spreading codes assigned to each user, and signals in which controlled signals of other users and thermal noises and so on are suppressed, are obtained. The obtained signals of users 1-n are sent to RAKE-combining devices 14-1˜14-N, respectively.
At the RAKE-combining devices 14-1˜14-N, RAKE-combining of signals of the users 1-n is performed to improve the quality of the signals. The signals of the users 1-n after the combining are sent to discrimination devices 15-1˜15-N and likelihood calculators 16-1˜16-N, respectively.
In the discrimination devices 15-1˜15-N, hard decision of the signals of the users 1-n is performed, and the signals after the decision are sent to the likelihood calculators 16-1˜16-N, respectively, and to a decision value buffer 17. The signals after the decision sent from the discrimination devices 15-1˜15-N are stored in the decision value buffer 17. That is, the signals of the users 1-n after the decision are stored in the decision value buffer 17.
The signals output from the RAKE-combining devices 14-1˜14-N, and the ones after the decision discrimination devices 15-1˜15-N are input to the likelihood calculators 16-1˜16-N. That is, the signals before and after the hard decision are input to the likelihood calculators 16-1˜16-N. In the likelihood calculators 16-1˜16-N, likelihood of all the symbols of each of signals of all the users 1-n are calculated. The above likelihood are indices denoting the probabilities of the above symbols, and the shorter the Euclidean distances between the signals before and after the hard decision are, and the greater the likelihood are. The signals on the above likelihood are sent to a likelihood buffer 18.
The signals on the above likelihood sent from the likelihood calculators 16-1˜16-N are stored in the likelihood buffer 18, and, sent to a ranking decision device 19.
In the ranking decision device 19, the ranking for each of all the symbols of signals of all the users is decided in order of greatness of the likelihood, based on the above likelihood. The above results are sent to a re-spreading device 20.
In the decision value buffer 17, a hard decision value of the highest symbol in the above ranking for stored signals is output as demodulation data, and sent to the re-spreading device 20, when the ranking is decided by the ranking decision device 19 as mentioned above.
In the re-spreading device 20, the hard decision value of the symbol with the greatest likelihoodent from the decision value buffer 17 is re-spread by the similar spreading code to the one at the above despreading to be sent to the subtraction device 21.
In the subtraction device 21, signals re-spread in the re-spreading device 20 are subtracted from the reception signals delayed by the delay device 12. The signals after the subtraction as mentioned above become the reception signals in which the symbol with the greatest likelihood and effects (interference) of the above symbol are completely cancelled, if the hard decision by the discrimination devices 15-1˜15-N is correct. The signals after the subtraction as described above in the subtraction device 21, that is, the signals after cancellation of interference are sent to the delay device 12 and the matched filters 13-1˜13-N through the switch 11.
Hereinafter, the above despreading, RAKE-combining, hard decision, and likelihood calculation are performed only for symbols under the effect of interference by the symbol with the greatest likelihood. Thereby, signals of all the users after the hard decision in the signals in which the symbol with the greatest likelihood and effects of the above symbol are removed, are stored in the decision value buffer 17. And, the signal concerning the likelihood of all the symbols of signals of all the users in the signals in which the symbol with the greatest likelihood and effects of the above symbol are removed, are stored in the decision value buffer 18.
Moreover, in the ranking decision device 19, re-ranking of all the symbols of signals of all the users is performed for updating, based on the likelihood. During the above ranking, the ranking is updated, excluding a symbol which is decided to have the greatest likelihood in the previous ranking operation. After updating of the ranking, the symbol with the greatest likelihood is output as demodulation data, as mentioned above, and subtracted from the signals sent from the delay device 12 by the subtraction device 21 after re-spreading by the re-spreading device 20. At this point, signals canceling the interference by each symbol with the first and the second high ranking based on the amounts of the likelihood from the reception signals are output from the subtraction device 21.
Thereafter, similar operations to those of the above are performed, until all the symbols of signals of all the users are demodulated. Thereby, demodulation data in which interference among signals is cancelled, are obtained.
However, in order to cancel interference from the reception signals, the above conventional device for canceling interference updates the ranking of all the symbols of signals of all the users, every demodulation of one symbol, that is, whenever a new likelihood is stored in the likelihood buffer, until all the symbols of signals of all the users are demodulated. As a result, there is a problem from a view point of cancellation processing speed, circuit configurations, and so on that there is less possibility to realize the above conventional device for canceling interference, as an enormous amount of operations are required to cancel effects by interference from the reception signals.